The present invention relates to a liquid crystal display apparatus and particularly concerns a liquid crystal display apparatus using an in plane switching method.
An active-matrix liquid crystal display apparatus has been known that has a TFT (TFT: THIN FILM TRANSISTOR) element as a switching element in a display region constituting a pixel.
As for the active-matrix liquid crystal display apparatus, a structure has been adopted in which a liquid crystal layer is inserted between a pair of substrates and is sandwiched therebetween. On one of the substrates (TFT substrate), TFT elements, pixel electrodes, electrodes and wiring for a scanning signal and a data signal, and terminals connecting the wiring and an external driving circuit are formed. On the other substrate (CF substrate), a color filter and opposing electrodes are formed. A twist nematic display method has been adopted in which a longitudinal electric field is applied to provide display. The longitudinal electric field is substantially perpendicular to a substrate surface.
Meanwhile, JP-A-6-160878 specification discloses a liquid crystal display apparatus using an in plain switching method as a method for improving a viewing angle and contrast that have been problems of a liquid crystal display apparatus. In this method, a common signal electrode is disposed on a TFT substrate instead of an opposing electrode disposed on a color filter substrate, and a voltage is applied between a comb-shaped pixel electrode and the common signal electrode. Thus, electric field components, which are substantially in parallel with a surface of the substrate, are used for providing display.
The pixel electrode and the common signal electrode may be made of a metallic electrode wiring material. Further, as disclosed in JP-A-9-73101 specification, the electrodes may be made of ITO (INDIUM TIN OXIDE), which is used as a transparent pixel electrode in a twist nematic display method.
In the above liquid crystal display apparatus using an in plain switching method, a leaked electric field that is unnecessary for its display occurs between signal wiring and the pixel electrode or the common signal electrode, the signal wiring being adjacent to the pixel electrode or the common signal electrode, as well as between the pixel electrode and the common signal electrode.
The signal wiring being adjacent to the electrodes includes scanning signal wiring, which is extended in x direction and is disposed in parallel in y direction, and data signal wiring, which is extended in y direction and is disposed in parallel in x direction.
It has been known that cross talk resulted from a leaked electric field drives liquid crystal thereon and causes leakage of light. Such leakage of light results in a defect in picture quality (smear) that is a line appearing along the signal wiring.
A means for solving the above problem is specifically disclosed in, for example, JP-A-6-202127 specification.
However, the liquid crystal display apparatus is configured such that a shield electrode for shielding an electric field is disposed next to the signal wiring and a reference potential is supplied to the shield electrode from the outside. Hence, a large current is charged and discharged to a capacity between the shield electrode and a signal electrode, thereby overloading a driving circuit.
Therefore, the liquid crystal display apparatus is large in power consumption or the driving circuit is too large in size. Further, connecting means is necessary for applying a potential to the shield electrode, resulting in a larger number of steps and a connecting defect.
Moreover, the above method is seriously disadvantageous as follows: an opening area on a pixel that contributes to display is reduced by the placement of the shield electrode, resulting in lower luminance of the liquid crystal display apparatus.
Means for solving the above problem is specifically disclosed in Japanese Patent Application No. 10-543713 (WO98/47044) specification.
A structure is adopted in which corresponding to image signal wiring which is adjacent to and in parallel with a comb-shaped pixel electrode, a reference electrode is formed while being completely superimposed on a plane via an organic insulating film formed entirely on a substrate. Thus, unnecessary electric lines of force that are generated from the data signal wiring are mostly terminated on the reference electrode.
Therefore, it is possible to solve the problem of cross talk that is caused by electric field leakage peculiar to an in plane switching method.
According to the above method, it is possible to eliminate the necessity for a shield electrode, which has been conventionally disposed on both sides of the data signal wiring or on an opposing substrate to reduce cross talk. Hence, the opening area on a pixel can be increased.
The liquid crystal layer on the reference electrode acts as a self-shielding layer and it is possible to eliminate a light shield film (black matrix), which has been conventionally disposed to cover a gap between the data signal wiring and the shield electrode. Hence, an aperture ratio of a pixel can be further increased.
Also, an organic insulating film, which is provided as an interlayer film entirely on the substrate, is small in permittivity as compared with an inorganic insulating film, and the organic insulating film can be readily increased in thickness as compared with the inorganic insulating film. Thus, even in the case where the data signal wiring is entirely covered with the reference electrode, it is possible to reduce a parasitic capacity of the wiring formed between the data signal wiring and the reference electrode.
Therefore, since a load is lighter with respect to the data signal wiring, a data signal has smaller delay in transmission through wiring and a signal voltage can be sufficiently charged to a display electrode.
Consequently, it is possible to downsize the driving circuit for driving the data signal wiring.
However, the above-mentioned conventional art has another problem as follows. As described above, when a voltage is applied to liquid crystal to provide display, a potential difference applied between the pixel electrode and the common signal electrode is used.
According to the above conventional art, the common signal electrode serving as a reference electrode is formed on the top layer via the organic insulating film, which is entirely formed on the substrate, to reduce a capacity between the data signal wiring and the common signal electrode. The pixel electrode is disposed on a lower layer, so that the organic insulating film is disposed on the pixel electrode.
Therefore, between the pixel electrode and the common signal electrode, the organic insulating film forms another capacity connected in series with liquid crystal.
Hence, a potential difference applied between the pixel electrode and the common signal electrode is partially reduced by the capacity connected in series with the liquid crystal layer.
As a result, between the pixel electrode and the common signal electrode, in consideration of the voltage drop, it is necessary to apply a potential difference at or more than a desired voltage to be applied to liquid crystal.
Namely, another problem arises as follows: a capacity component connected in series with the liquid crystal layer is reduced due to the presence of the organic insulating film, which is formed to decrease a capacity between the data signal wiring and the common signal electrode, resulting in larger voltage drop caused by a capacity connected in series with liquid crystal. Consequently, liquid crystal requires a larger driving voltage.
An increase in driving voltage raises power consumption, which is inappropriate particularly for a portable liquid crystal display apparatus.
Further, when a driving voltage increases, an inexpensive low-voltage driver is not applicable. Hence, it is not possible to provide a liquid crystal display apparatus at low cost.
According to the above conventional art, in the liquid crystal display apparatus using an in plain switching method, the common signal electrode serving as a reference electrode for shielding an electric field is superimposed on the signal wiring via the interlayer insulating film. In this structure, it is difficult to reduce a parasitic capacity formed on a superimposed part of the signal wiring and the common signal electrode and simultaneously to increase a capacity connected in series with liquid crystal between the pixel electrode and the common signal electrode.
In contrast, when the pixel electrode is disposed on the organic insulating film, that is, on the top layer where the common signal electrode is disposed, it is possible to avoid the above-mentioned increase in driving voltage (Japanese Patent Application No. 10-543713 (WO98/47044) specification).
However, since the pixel electrode and the common signal electrode are disposed on the same layer, a short circuit is more likely to occur due to a patterning defect and so on between the pixel electrode and the common signal electrode.
Accordingly, a large space is required between the pixel electrode and the common signal electrode, resulting in a limit on the alignment of the pixel electrode and the common signal electrode.
Besides, in order to connect the pixel electrode and the source electrode of the TFT, it is necessary to form another through hole on the organic insulating film. Hence, a point defect is more likely to occur due to a connecting defect and so on. Also, since the through hole does not act as an opening, an aperture ratio decreases accordingly.
An object of the present invention is to provide an in plain switching liquid crystal display apparatus, in which a common signal electrode serving as a reference electrode for shielding an electric field is superimposed on at least one of data signal wiring and scanning signal wiring via an interlayer insulating film, characterized in that it is possible to reduce a parasitic capacity of signal wiring formed on a superimposed part of the data signal wiring or the scanning signal wiring and the common signal electrode and to increase a capacity connected in series with liquid crystal between a pixel electrode and a common signal electrode.
In order to attain the above object, the present invention is characterized by providing the following measures for a liquid crystal display apparatus, which includes a pair of substrates and a liquid crystal layer sandwiched between the substrates, the pair of substrates including a first substrate having a plurality of scanning signal wiring, a plurality of data signal wiring intersecting the scanning signal wiring in a matrix form, and a plurality of thin-film transistors formed on the intersections, wherein at least a single pixel is formed in each region surrounded by the plurality of scanning signal wiring and data signal wiring, each pixel includes a common signal electrode, which is connected to a plurality of pixels via common signal wiring, and a pixel electrode connected to the corresponding thin-film transistor, and a voltage applied to the common signal electrode and the pixel electrode generates in the liquid crystal layer an electric field having a parallel component predominantly to the first substrate.
(1) The common signal electrode and at least one of the data signal wiring and the scanning signal wiring are partially superimposed onto each other via an interlayer insulating film, and the superimposed part forms a capacity. At least one of insulating films included in the interlayer insulating film is selectively formed at least on a part of a region on the pixel electrode and at least in the superimposed part of the common signal electrode and at least one of the data signal wiring and the scanning wiring.
(2) The common signal electrode and at least one of the data signal wiring and the scanning signal wiring are partially superimposed onto each other via an interlayer insulating film. The superimposed part selectively forms a capacity. When SA denotes Equation 9 and SB denotes Equation 10 (mxe2x89xa71), SA less than SB is satisfied, where n indicates the number of layers of the insulating films included in the interlayer insulating film, xcex5K indicates a permittivity of an insulating film on k layer, dK indicates a film thickness, m indicates the number of layers of insulating films disposed between the pixel electrode and a first alignment film disposed on the pixel electrode on at least a part of a region on the pixel electrode, xcex5L indicates a permittivity of an insulating film of L layer, dL indicates a film thickness, and xcex5LC indicates a permittivity of liquid crystal in a perpendicular direction to a director of liquid crystal.                     1                              ∑                          k              =              1                        n                    ⁢                      xe2x80x83                    ⁢                                    d              k                                      ϵ              k                                                          (        9        )                                1                              (                                          ∑                                  L                  =                  1                                m                            ⁢                              xe2x80x83                            ⁢                                                d                  L                                                  ϵ                  L                                                      )                    +                                                                      ∑                                      k                    =                    1                                    n                                ⁢                                  xe2x80x83                                ⁢                                  d                  k                                            -                                                ∑                                      L                    =                    1                                    m                                ⁢                                  xe2x80x83                                ⁢                                  d                  L                                                                    ϵ              LC                                                          (        10        )            
(3) The common signal electrode and at least one of the data signal wiring and the scanning signal wiring are partially superimposed onto each other via an interlayer insulating film. The superimposed part forms a capacity. When SA denotes Equation 11 and SB denotes Equation 12 (mxe2x89xa71), SA less than SB is satisfied, where n indicates the number of layers of the insulating films included in the interlayer insulating film, xcex5K indicates a permittivity of an insulating film on k layer, dK indicates a film thickness, m indicates the number of layers of insulating films disposed on the pixel electrode at least in a part of a region on the pixel electrode, xcex5L indicates a permittivity of an insulating film on L layer, dL indicates a film thickness, and xcex5LC indicates a permittivity of liquid crystal in parallel with a director of liquid crystal.                     1                              ∑                          k              =              1                        n                    ⁢                      xe2x80x83                    ⁢                                    d              k                                      ϵ              k                                                          (        11        )                                1                              (                                          ∑                                  L                  =                  1                                m                            ⁢                              xe2x80x83                            ⁢                                                d                  L                                                  ϵ                  L                                                      )                    +                                                                      ∑                                      k                    =                    1                                    n                                ⁢                                  xe2x80x83                                ⁢                                  d                  k                                            -                                                ∑                                      L                    =                    1                                    m                                ⁢                                  xe2x80x83                                ⁢                                  d                  L                                                                    ϵ              LC                                                          (        12        )            
(4) The common signal electrode and at least one of the data signal wiring and the scanning signal wiring are partially superimposed onto each other via an interlayer insulating film. The superimposed part forms a capacity. No insulating film exists between the first alignment film and the pixel electrode which are disposed on the first substrate. When SA denotes Equation 13 and SB denotes Equation 14, SA less than SB is satisfied, where n indicates the number of layers of the insulating films included in the interlayer insulating film, xcex5K indicates a permittivity of an insulating film on k layer, dK indicates a film thickness, and xcex5LC indicates a permittivity in a perpendicular direction to a director of liquid crystal.                     1                              ∑                          k              =              1                        n                    ⁢                      xe2x80x83                    ⁢                                    d              k                                      ϵ              k                                                          (        13        )                                          ϵ          LC                                      ∑                          k              =              1                        n                    ⁢                      xe2x80x83                    ⁢                      d            k                                              (        14        )            
(5) The common signal electrode and at least one of the data signal wiring and the scanning signal wiring are partially superimposed onto each other via an interlayer insulating film. The superimposed part forms a capacity. No insulating film exists between the first alignment film and the pixel electrode which are disposed on the first substrate. When SA denotes Equation 15 and SB denotes Equation 16, SA less than SB is satisfied, where n indicates the number of layers of the insulating films included in the interlayer insulating film, xcex5k indicates a permittivity of an insulating film on k layer, dK indicates a film thickness, and xcex5LC indicates a permittivity in parallel with a director of liquid crystal.                     1                              ∑                          k              =              1                        n                    ⁢                      xe2x80x83                    ⁢                                    d              k                                      ϵ              k                                                          (        15        )                                          ϵ          LC                                      ∑                          k              =              1                        n                    ⁢                      xe2x80x83                    ⁢                      d            k                                              (        16        )            
With the structures (1) to (5), even when reducing a parasitic capacity of signal wiring that is formed on a superimposed part of the data signal wiring or the scanning signal wiring and the common signal electrode, it is possible to independently increase a capacity connected in series with liquid crystal between the pixel electrode and the common signal electrode, thereby suppressing an increase in driving voltage.
To be specific, as described in (1), the interlayer insulating film formed on a superimposed part of the wiring and the common signal electrode, that is, the interlayer insulating film formed for reducing a parasitic capacity of wiring is not formed on the pixel electrode. Namely, the insulating film is formed selectively. Thus, it is possible to arbitrarily reduce a parasitic capacity of wiring separately from a capacity connected in series with a liquid crystal layer by increasing a thickness of the interlayer insulating film or changing the structure of the interlayer insulating film.
As described in (1) to (5), regarding a reduction in driving voltage, the insulating film on a region having the pixel electrode, that is, the insulating film forming a capacity connected in series with the liquid crystal layer is removed selectively.
Thus, liquid crystal exists on a selectively removed region. In order to obtain the effect of reducing a driving voltage, value obtained from Equation 18 needs to be larger than that Equation 17, where j indicates the number of layers of insulating films included in a selectively removed insulating film, xcex5I indicates a permittivity of an insulating film on i layer, dI indicates a film thickness, and xcex5LC indicates a permittivity of liquid crystal.
Here, xcex5LC is a permittivity in parallel with a director of liquid crystal when xcex94xcex5 is positive liquid crystal, and xcex5LC is a permittivity in a perpendicular direction to a director of liquid crystal when xcex94xcex5 is negative liquid crystal. Namely, a permittivity is obtained from the lower substrate to the upper substrate when a voltage is applied to the liquid crystal layer.                     1                              ∑                          i              =              1                        j                    ⁢                      xe2x80x83                    ⁢                                    d              i                                      ϵ              i                                                          (        17        )                                          ϵ          LC                                      ∑                          i              =              1                        j                    ⁢                      xe2x80x83                    ⁢                      d            i                                              (        18        )            
The effect of reducing a driving voltage can be obtained only when the above equations are satisfied. Since xcex5LC is 7 or more in a liquid crystal layer of a conventional liquid crystal display apparatus, considering the selectively formed insulating film is made of a material such as silicon nitride (xcex5=6 to 7) and silicon oxide (xcex5=3 to 4), in most cases, the selective removal of the insulating film makes it possible to increase a capacity connected in series with liquid crystal between the pixel electrode and the common signal electrode, thereby reducing a driving voltage.
Further, as a combination of the above methods, to be specific, the interlayer insulating film formed on a superimposed part of the wiring and the common signal electrode is configured as a lamination including a part of an insulating film serving as a gate insulating film, a part of an insulating film serving as a surface protecting film of the thin-film transistor, and another new insulating film. The new insulating film is formed selectively in a region on the pixel electrode. Moreover, an insulating film used in the conventional structure is formed selectively in a region on the pixel electrode and is formed selectively in the same region as the new insulating film.
Next, the following will discuss more specific structures for realizing the above structure.
(6) In the liquid crystal display apparatus described in any one of (1) to (5), regarding the interlayer insulating film and the insulating film disposed between the first alignment film and the pixel electrode that are formed on the first substrate at least on a part of a region on the pixel electrode, a difference is made at least in one of the number of layers of the insulating films, a film thickness of a material for forming the layer, and a permittivity of a material for forming the layer.
(7) In the liquid crystal display apparatus described in any one of (1) to (6), the interlayer insulating film is composed of a single layer and the single layer is selectively formed at least on a part of a region on the pixel electrode.
(8) In the liquid crystal display apparatus described in (7), the interlayer insulating film is a part of a first insulating film serving as the gate insulating film of the thin-film transistor or a part of a second insulating film serving as a surface protecting film of the thin-film transistor.
(9) In the liquid crystal display apparatus described in (7), the interlayer insulating film is a third insulating film other than the first insulating film serving as the gate insulating film of the thin-film transistor or the second insulating film serving as a surface protecting film of the thin-film transistor.
(10) In the liquid crystal display apparatus described in any one of (1) to (6), the interlayer insulating film is composed of two layers, and at least one of the layers is selectively formed at least on a part of a region on the pixel electrode.
(11) In the liquid crystal display apparatus described in (10), the interlayer insulating film is composed of two layers including a part of the first insulating film serving as the gate insulating film of the thin-film transistor or a part of the second insulating film serving as a surface protecting film of the thin-film transistor.
(12) In the liquid crystal display apparatus described in (10), in the interlayer insulating film, one of the layers is a part of the first insulating film serving as the gate insulating film of the thin-film transistor or a part of the second insulating film serving as a surface protecting film of the thin-film transistor, and the other layer is a third insulating film other than the first insulating film and the second insulating film. The third insulating film is selectively formed at least on a part of a region on the pixel electrode.
(13) In the liquid crystal display apparatus described in any one of (1) to (6), the interlayer insulating film is composed of three or more layers. At least one of them is selectively formed at least on a part of a region on the pixel electrode.
(14) In the liquid crystal display apparatus described in (13), the interlayer insulating film includes all of a part of the first insulating film serving as the gate insulating film of the thin-film transistor, a part of the second insulating film serving as a surface protecting film of the thin-film transistor, and the third insulating film other than the first insulating film and the second insulating film. The third insulating film is selectively formed at least on a part of a region on the pixel electrode.
(15) In the liquid crystal display apparatus described in any one of (1) to (14), at least on a part of a region on the pixel electrode, the pattern of the interlayer insulating film, which is formed selectively on a superimposed part of the common signal electrode and at least one of the data signal wiring or the scanning signal wiring, is formed according to the pattern of the data signal wiring or the scanning signal wiring.
(16) In the liquid crystal display apparatus described in (15), when a width of the data signal wiring is WDL, a width of the common signal electrode formed on a part superimposed with the data signal wiring is WCOM1, and a width of the interlayer insulating film selectively formed according to the pattern of the data signal wiring is WIS01, WDL less than WIS01 less than WCOM1 and WDL greater than 0 or WDL less than WCOM1 less than WIS01 and WDL greater than 0 are established.
(17) In the liquid crystal display apparatus described in (15), when a width of the scanning signal wiring is WGL, a width of the common signal electrode formed on a part superimposed with the scanning signal wiring is WCOM2, and a width of the interlayer insulating film selectively formed according to the pattern of the scanning signal wiring is WIS02, WGL less than WIS02 less than WCOM2 and WGL greater than 0 or WGL less than WCOM2 less than WIS02 and WGL greater than 0 are established.
(18) In the liquid crystal display apparatus described in any one of (1) to (14), on the interlayer insulating film formed on a superimposed part of the common signal electrode and the data signal wiring, at least a part of the insulating film, which is formed at least on a part of a region on the pixel electrode, is selectively removed or reduced in thickness.
(19) In the liquid crystal display apparatus described in (18), at least a part of the insulating film, which is formed at least on a part of a region on the pixel electrode, is selectively removed or reduced in thickness according to the pattern of the pixel electrode.
(20) In the liquid crystal display apparatus described in (19), when a width of the pixel electrode is WPX and a width of the interlayer insulating film selectively removed or reduced in thickness according to the pattern of the pixel electrode is WIS03, WIS03 less than WPX and WIS greater than 0 are established.
(21) In the liquid crystal display apparatus described in any one of (1) to (20), at least in a region other than an exposed region for connecting terminals, a fourth insulating film is formed so as to cover at least the pixel electrode and the common signal electrode.
With this arrangement, coating and protection can be provided on the surfaces of the pixel electrode and the common signal electrode, thereby preventing a side effect such as mutual contamination, which is caused by contact between an electrode material and liquid crystal.
(22) In the liquid crystal display apparatus described in any one of (1) to (7), (9), (10), (12), (13), and (15) to (21), the second insulating film serving as the surface protecting film of the thin-film transistor is omitted.
The third insulating film substitutes for the surface protecting film of the thin-film transistor. Thus, it is possible to omit the step of forming the surface protecting film of the thin-film transistor, thereby simplifying the manufacturing process.
(23) In the liquid crystal display apparatus described in any one of (7), (9), (10), (12), to (22), the third insulating film and the fourth insulating film are coating type insulating films.
(24) In the liquid crystal display apparatus described in (23), the coating type insulating film is formed by a method such as printing and spin coating method, and the coating type insulating film is an organic resin insulating film or an insulating film containing Si.
(25) In the liquid crystal display apparatus described in (23) or (24), the coating type insulating film used as the third insulating film is a photo-image type.
(26) In the liquid crystal display apparatus described in any one of (10), and (12) to (25), the first insulating film serving as the gate insulating film of the thin-film transistor, the second insulating film serving as the surface protecting film of the thin-film transistor, or a laminated film of the first insulating film and the second insulating film is collectively processed in a self-aligning manner by using the third insulating film pattern as described above. At least on a part of a region on the pixel electrode, the first insulating film, the second insulating film, or the laminated film of the first and second insulating films is formed selectively.
(27) In the liquid crystal display apparatus described any one of (7), (9), (10), and (12) to (26), the third insulating film has a thickness of 0.5 to 4.0 xcexcm.
(28) In the liquid crystal display apparatus described any one of (7), (9), (10), and (12) to (27), the third insulating film has a permittivity of 1.5 to 6.5.
(29) In the liquid crystal display apparatus described in (21), the coating type insulating film used as the fourth insulating film has a thickness of 0.1 to 0.5 xcexcm.
(30) In the liquid crystal display apparatus described in any one of (1) to (17) and (21) to (29), a fifth insulating film is selectively formed with a permittivity of 7.0 or more so as to fill and flatten a step height region appearing due to the interlayer insulating film which is formed selectively.
(31) In the liquid crystal display apparatus described in any one of (18), (19), and (20), the fifth insulating film is selectively formed with a permittivity of 7.0 or more so as to fill and flatten a step height region, which is formed by selectively removing or reducing in thickness at least a part of an insulating film formed at least on a part of a region on the pixel electrode.
With the structures of (30) and (31), it is possible to reduce a driving voltage regardless of a permittivity of the liquid crystal layer.
Further, it is possible to fill and flatten a step height formed by selective formation or selective removal of the insulating film.
(32) In the liquid crystal display apparatus described in any one of (1) to (31), the common signal wiring is formed by extending the common signal electrode on the same layer as the common signal electrode.
(33) In the liquid crystal display apparatus described in any one of (1) to (32), the common signal wiring is formed on the same layer as the scanning signal wiring or the data signal wiring, and the common signal wiring and the common signal electrode are connected to each other via a through hole, which is opened on the interlayer insulating film.
(34) In the liquid crystal display apparatus described in any one of (1) to (33), the pixel electrode is composed of a transparent conductive film made of indium oxide such as indium tin oxide (ITO), indium zinc oxide (IZO), and indium germanium oxide (IGO).
(35) In the liquid crystal display apparatus described in (34), the pixel electrode is composed of a transparent conductive film made of polycrystalline indium oxide.
(36) In the liquid crystal display apparatus described in any one of (1) to (35), at least a part of the common signal electrode is composed of a transparent conductive film made of indium oxide such as indium tin oxide (ITO), indium zinc oxide (IZO), and indium germanium oxide (IGO).
(37) In the liquid crystal display apparatus described in (36), at least a part of the common signal electrode includes the transparent conductive film made of amorphous indium oxide.
(38) In the liquid crystal display apparatus described in any one of (34) to (37), a normally black mode is set in which black display is provided when no electric field is generated between the pixel electrode and the common signal electrode.
According to the present invention, it is possible to provide a liquid crystal display apparatus of an in plane switching that has a large pixel aperture ratio and a high luminance without causing any signal delay of a wiring or increase of a driving voltage.